VideoBlockEditor:Poster Modal

Build 1715 on 22-03-SC-refactor-dialogs-to-common
1200px
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Baseline History
Build 171571d332d
Current build. There are unaccepted changes, so build 1698 is still the baseline.
Build 1691de1cb44
Accepted by Tataihono Nikora.
Build 1689adc9219
Accepted by Tataihono Nikora.
Build 1637a265444
Accepted by Siyang Cao.
Commenting is disabled because there's a newer build on this branch